The square bag is shown empty by turning it inside out. It is then turned right side out, and one by one, four squares of material are placed into the empty bag. Attention is drawn to the fact that the bag is square, and that you have placed four squares inside it. The bag is now turned inside out again, however this time inside is one round circle of cloth.
The audience is told that the four cloths, which were once square, have now become one round cloth. The bag is turned inside out a second time and actually has turned "round". This is a delightful baffling effect.
Perfect for pre-school shows where the children are learning shapes.